How to make simple JSON object using C# string Builder

I am new at json, I want to make jquery ajax call to a asp.net webmethod to get back simple JSON object using string builder

[WebMethod]
public static string GetmyJSON()
{
    StringBuilder sb = new StringBuilder();       
    sb.Append("{firstname: \"Manas\",").Append("lastname : \"Tunga\"").Append("}");
    return sb.ToString();    

}

In my Clint side code i have written

.ajax({

        type: "POST",
        url: "simplePage.aspx/GetmyJSON",           
        data: "{}",
        contentType: 'application/json; charset=utf-8',
        dataType: 'json',

        success: function (data) {

            alert(data.d.firstname);   //only showing undefined     insted of Manas


        } // end of sucess

    }); // End of ajax

But My alert message showing undefined instead of Manas why, Where I am doing wrong isn't possible to return a json object using string builder ??? if possible how to write it.